1. Field of the Invention
The present invention relates to directional drilling and, more specifically, to so-called navigational drilling, wherein a bottomhole assembly including a downhole motor of the positive-displacement or turbine type is employed to drill both linear and nonlinear segments of a borehole to follow a desired path. In a preferred embodiment, the invention permits continuous rotation of a string of drill pipe above the bottomhole assembly while compensating the bottomhole assembly for reactive torque forces induced in the assembly by the downhole motor and either maintaining the bottomhole assembly in a rotationally static position, rotating the bottomhole assembly, or permitting the bottomhole assembly to rotate in a controlled fashion independently of the drill string.
2. State of the Art
Navigational drilling is a commercially viable technology employed in oil and gas exploration. Commercial navigational drilling bottomhole assemblies fielded in the past ten years have employed turbines or positive-displacement (Moineau principle or, most recently, vane-type) motors (hereinafter generically termed "downhole motors" or "motors") secured to the end of a drill string extending to the rig floor. A single or multiple-bend sub or housing is employed, preferably below the motor power section, to angle the motor drive shaft and hence the axis of the drill bit secured to the shaft, at a slight angle (generally on the order of 4.degree. or less) to the axis of the motor and thus to the drill string immediately above the motor. Other techniques employed in the past to angle or laterally bias the bit with respect to the string axis include the use of an angled bearing sub at the motor and the use of one or more eccentric stabilizers. Exemplary patents disclosing bottomhole assemblies of the aforementioned types and others are disclosed in U.S. Pat. Nos. 5,343,967; 4,807,708; 5,022,471; 5,050,692; 4,610,307; and Re 33,751. Such assemblies may be termed generically to include "deflection devices" of any type known in the art, the term deflection device as used herein meaning an element or combination of elements in a bottomhole assembly for angling the drill bit axis with respect to either the motor, the entire bottomhole assembly, or the drill string for directional (oriented) drilling purposes, or that cause a bias in the drill bit side loading such that directional drilling is achieved through the side-cutting action of the drill bit under the influences of the lateral bias.
Steerable bottomhole assemblies using downhole-adjustable bent subs or housings as well as assemblies using extendable steering pads on one or multiple sides of the assembly have also been disclosed, but are not in widespread or even limited commercial use to the knowledge of the inventors. Moreover, such assemblies are complex, expensive to build, and currently of questionable reliability.
Returning to the fixed-angle (non-adjustable while deployed in the wellbore) type of bottomhole navigational drilling assembly, it should be noted that the downhole drilling motor is in continuous operation to rotate the drill bit at the end of the string, whether a straight or a curved borehole trajectory is desired. When it is desired to drill straight ahead, right-hand (clockwise, looking down) drill string rotation via a rotary table or top drive is superimposed upon the right-hand rotation of the bit effected by the motor. In such a manner, the slight angle of deviation between the bit axis and the motor or string axis, or the bias in drill bit side loading, is compensated and rendered neutral with respect to influence on wellbore trajectory, although in actual practice the "straight" borehole may spiral or corkscrew about the intended "straight" path by virtue of other influences. When a curved or nonlinear borehole segment is to be drilled, rotation of the string is stopped, and the rotational orientation angle of the output shaft and drill bit (tool face orientation or TFO) is adjusted to a desired heading by incremental drill string rotation effected from the surface, which is monitored by a steering or directional-orientation tool (DOT) or via a measurement-while-drilling (MWD) assembly, the sensors of such instruments being placed as close as possible to the motor for accuracy.
While navigational drilling systems employing apparatus and the basic methods as described above have been commercially successful, at least one major drawback remains. Specifically, when in the directional or oriented drilling mode, the stationary drill string above the motor results in greatly increased friction between the drill string and the wall of the borehole along the longitudinal wellbore axis, which phenomenon is responsible for "slip-stick" behavior of the string wherein the string may alternately seize and release in the borehole, both axially and rotationally. When string angular or rotational orientation is attempted from the rig floor, this slip-stick behavior may cause a correct TFO to deviate as frictional forces and reactive torque reduce or increase immediately after a reading is taken. Moreover, the drill string may actually "wind-up" while it is being rotated, the extent of such wind-up varying with the reactive (left-hand) torque from the motor and with the angular or rotational elasticity or compliance of the drill string. When the string relaxes and unwinds, TFO again may be vastly altered.
It has also been proposed to employ bottomhole assemblies including downhole motors at the end of coiled tubing strings, given the great rig time advantage coiled tubing offers over the use of conventional drill pipe joints. However, coiled tubing cannot be rotated from the surface, even to a limited degree for bottomhole assembly orientational purposes and certainly not for rotating the bottomhole assembly on a continuing basis. Therefore, a fixed-angle or fixed-bias bottomhole assembly cannot be used when the ability to drill both straight ahead and on a curve is desired. A state-of-the-art coiled tubing-run bottomhole assembly must, as a consequence, include another type of orienting mechanism to vary the orientation of the bit axis between coincident with and angled with respect to the motor or string. One such apparatus is disclosed in U.S. Pat. No. 5,311,952, issued on May 17, 1994 to Eddison et al. In addition to the problem of angular adjustment, bottomhole assemblies run on coiled tubing may present control problems for the reactive torque generated by the downhole motor, which at its maximum (incipient motor stall) cannot be effectively accommodated by the coiled tubing in the same manner as with relatively more torsionally rigid and robust drill pipe.
In short, state-of-the-art drill pipe-run and coiled tubing-run navigational drilling systems each possess some disadvantages and limitations, rendering their performance less than optimum.